package algotithm.leetcode.listnode.testt234;

import algotithm.leetcode.listnode.ListNode;

/**
 * @author zhouyanxiang
 * @Date 2021-02-2021/2/18-19:46
 * @URL https://leetcode-cn.com/problems/palindrome-linked-list/
 * @Title 234. 回文链表
 */
public class Solution {

    public static void main(String[] args) {
        ListNode t1 = new ListNode(1);
        ListNode t2 = new ListNode(2);
        ListNode t3 = new ListNode(1);
        t1.next = t2;
        t2.next = t3;

        Solution solution = new Solution();
        boolean palindrome = solution.isPalindrome(t1);
        System.out.println(palindrome);
    }

    public boolean isPalindrome(ListNode head) {
        if (head == null) {
            return true;
        }

        // 找到前半部分链表的尾节点并反转后半部分链表
        ListNode firstHalfEnd = endOfFirstHalf(head);
        ListNode secondHalfStart = reverseList(firstHalfEnd.next);

        // 判断是否回文
        ListNode p1 = head;
        ListNode p2 = secondHalfStart;
        boolean result = true;
        while (result && p2 != null) {
            if (p1.val != p2.val) {
                result = false;
            }
            p1 = p1.next;
            p2 = p2.next;
        }

        // 还原链表并返回结果
        firstHalfEnd.next = reverseList(secondHalfStart);
        return result;
    }

    private ListNode reverseList(ListNode head) {
        ListNode prev = null;
        ListNode curr = head;
        while (curr != null) {
            ListNode nextTemp = curr.next;
            curr.next = prev;
            prev = curr;
            curr = nextTemp;
        }
        return prev;
    }

    private ListNode endOfFirstHalf(ListNode head) {
        ListNode fast = head;
        ListNode slow = head;
        while (fast.next != null && fast.next.next != null) {
            fast = fast.next.next;
            slow = slow.next;
        }
        return slow;
    }
}


